Full Chapter Overview
1.1 Variables
What are Variables?
A variable is a placeholder for an unknown or general value. It can represent a single number, multiple numbers, or a general category of elements. Think of variables as mathematical placeholders—they temporarily stand in for values we do not yet know.
Why Use Variables?
Unknown Values:
When solving equations, a variable represents an unknown. Example: "Is there a number that, when doubled and added to 3, equals its square?" This can be written as: 2x + 3 = x²
General Statements:
Variables help express general rules. Example: Instead of saying, "The square of any even number is even," we generalize: For any even integer n, n² is even.
Types of Mathematical Statements
Mathematical statements often fall into these categories:
Universal Statements: Apply to all elements in a set. Example: "All positive numbers are greater than zero."
Conditional Statements: Express if-then logic. Example: "If a number is divisible by 18, then it is divisible by 6."
Existential Statements: Assert that at least one example exists. Example: "There exists a prime number that is even."
Universal Conditional Statements: A universal conditional statement is a statement that is both universal and conditional. Example: For every animal a, if a is a dog, then a is a mammal.
Universal Existential Statements: A universal existential statement is a statement that is universal because its first part says that a certain property is true for all objects of a given type, and it is existential because its second part asserts the existence of something. Example: Every real number has an additive inverse.
Existential Universal Statements: An existential universal statement is a statement that is existential because its first part asserts that a certain object exists and is universal because its second part says that the object satisfies a certain property for all things of a certain kind. Example: There is a positive integer that is less than or equal to every positive integer.
How to Use Variables
Choose a letter (e.g., x, y, z, n, m) to represent the variable.
Clearly define what the variable represents (e.g., "Let x be a real number").
Substitute the variable into equations or expressions.Example: Solve for x:
3x + 5 = 14
Subtract 5 from both sides: 3x = 9
Divide by 3: x = 3
Summary
Variables are essential for expressing mathematical ideas precisely. They help us solve problems and make general statements efficiently.
1.2 The Language of Sets
What is a Set?
A set is a collection of distinct objects, called elements or members. The order of elements in a set does not matter.
Representing Sets
Set-Roster Notation: Listing elements inside curly braces.Example: A = {1, 2, 3}
Set-Builder Notation: Describes properties elements must satisfy.Example: B = {x | x is a positive real number}
Special Sets of Numbers
R: The set of all real numbers (rational + irrational numbers).
Z: The set of all integers {..., -2, -1, 0, 1, 2,...}.
Q: The set of all rational numbers (fractions).
N: The set of all positive integers {1, 2, 3,...}.
Empty Set: The empty set is a set that has no elements. It is denoted by the symbol ∅ or {}.
Relationships Between Sets
Subset (⊆): A ⊆ B means every element in A is in B.
Proper Subset (⊂): A ⊂ B means A is a subset of B but not equal to B.
Equality (=): Two sets are equal if they contain the exact same elements.
Operations on Sets
Union (∪): A ∪ B contains all elements in A or B (or both).
Intersection (∩): A ∩ B contains elements that are in both A and B.
Difference (A - B): Elements in A but not in B.
Cartesian Product (A × B): The set of all ordered pairs (a, b), where a is in A and b is in B.Example: If A = {1, 2} and B = {x, y}, then: A × B = {(1, x), (1, y), (2, x), (2, y)}
Distinction between ∈ and ⊆
∈: The symbol ∈ means "is an element of". It is used to show that an object is a member of a set. Example: 1 ∈ {1, 2, 3}
⊆: The symbol ⊆ means "is a subset of". It is used to show that one set is contained in another set. Example: {1, 2} ⊆ {1, 2, 3}
Summary
Sets are foundational in mathematics. Understanding set notation, special sets, and set operations is crucial for logical reasoning.
1.3 The Language of Relations and Functions
What is a Relation?
A relation between two sets A and B is a set of ordered pairs (a, b), where a is in A and b is in B. This defines a relationship between elements of the sets.
Functions
A function is a special relation where each element in A is related to exactly one element in B.
Function Machines
Functions can be thought of as machines that take an input, process it, and produce a single output.
Arrow Diagram of a Relation
An arrow diagram is a visual way to represent a relation between two sets. The elements of the sets are represented as points, and arrows are drawn between the points to indicate the relation.
Types of Functions
Injective Function: A function is injective if each element in the co-domain is mapped to by at most one element in the domain. (ie: R = R)
each output has a maximum of one input
Surjective Function: A function is surjective if each element in the co-domain is mapped to by at least one element in the domain. (ie: R = R²)
each output has at least one input but can have more than one
Bijective Function: A function is bijective if it is both injective and surjective. (ie: R = R³)
This means that for every element in the co-domain, there exists a unique element in the domain that maps to it, ensuring a perfect one-to-one correspondence between the two sets.
Summary
Relations define general connections between sets, while functions ensure each input maps to exactly one output.
1.4 The Language of Graphs
What is a Graph?
A graph consists of:
Vertices (Nodes): Points representing elements.
Edges: Lines connecting vertices.
Types of Graphs
Directed Graph: Edges have a direction (arrows).
Undirected Graph: Edges have no direction.
Weighted Graph: Edges have a weight associated with them.
Graphs as Relations
Graphs can visually represent relations by using:
Vertices for elements.
Edges to indicate relations.
Ways to Represent a Graph
Adjacency Matrix: A matrix that stores the weights of the edges between vertices.
Adjacency List: A list that stores the neighbors of each vertex.
Summary
Graphs provide a visual way to represent relationships, making patterns easier to analyze.